The Concept of the Fatigue Potential of Alloy

FATIGUE

Fatigueis a type of failure of a material, occurring under alternating loads.

Most of the failures of machine details are caused by fatigue.

Fatigue lifeis the number of cycling stresses, causing failure of the material.

Frequency of these stresses is not important.

Fatigue limitis the maximum value of repeatedly applied stress that the material can withstand after an infinite number of cycles (10-20 mln. Cycles in practice).

Fatigue strength at N cyclesis the load, producing the material fracture after N cycling applications of the load.

Fatigue limit of a material is much lower, than its ultimate tensile strength.

Fatigue tests are carried out in the Wöhler-type machine, schematically shown in the picture.

The rotating specimen in form of a cantilever is driven by an electric motor. The specimen is loaded by the force F, applied to the ball bearing, mounted on the end of the specimen.

Since the force direction does not change, the direction of the stress applied to the specimen will be reversed each 180º of the shaft rotation.

This scheme provides cycling loading of the specimen, presented in the equivalent scheme.

To find the fatigue limit the fatigue test is repeated at different loads.

The tests results are presented in form of S-N curve (stress vs. number of cycles):

Fatigue fracture is characterized by presence of two different types of the surface:

One part is smooth and discolored with ripple-like marks, indicating slow crack growth from the center of the crack formation. Another part of the surface has coarse crystalline appearance resulted from the final catastrophic crack propagation.

The following factors affect fatigue fracture:

§ Surface factor

Fatigue cracks form and initiate on the specimen surface therefore hardened and smooth surface (without stress concentrations - notch, flaw) will increase the fatigue limit.

§ Compressive stress

Compressive stresses, produced in the specimen surface by Shot peening, cold work or heat treatment result in considerable increase of fatigue limit.

§ Micro-structure defects

Non-metallic inclusions and other micro-defects may initiate formation of fatigue cracks.

§ Environmental factor

Fatigue in the presence of corrosive environment (Corrosion fatigue) occurs at lower cycling stresses and after lower number of cycles.

The Concept of the Fatigue Potential of Alloy

Materials fail by the initiation cracks from their most serious defects. If these defects are eliminated by careful and appropriate processing, then the performance of the material is likely to be improved. Strength and durability in service are then limited by the next most serious family of defects, and so on. When each family of defects has been removed, the material will fail from metallurgical features such as the large intermetallic compounds or other brittle phases which might be present. Finally, if these in turn are removed, we have the ultimate resistance to failure as dictated by the metallurgy of the alloy, for instance the ability of the alloy to resist slip, and once slip has been initiated, the slip distance as dictated by the grain size enshrined in the Hall-Petch relation.

Thus in the hierarchy of features which initiate cracks which lead to failure, much exemplary scientific work has been carried out on the metallurgical aspects of failure initiation. However, to understand the causes of failure in most manufactured components, particularly cast products, it is essential to identify those features which might have precedence in the hierarchy. In general such features have been overlooked because of the statistical problem of finding large features in small specimens generally used in metallurgical studies. It is clearly easy to concentrate on those features such as gram size and subgram structure which are the outstanding features of microscopical examination. However, on this scale, features measured in areas of mm² or cm² are often visible to the naked eye because they are so thin, but also remain undetected by microscopic examination because they are statistically rare in such small samples. Regrettably, they are not rare on the scale of the whole cast component, and are likely therefore to be the very features which cause failure in service.

For this reason, work has been carried out at The University of Birmingham to determine the nature of these elusive major faults at the top of the hierarchy of potential initiators. Fatigue studies on a commonly used aluminium cast alloy, Al-7Si-0.4Mg, were carried out. The alloy was prepared from good quality starting materials, carefully melted and cast and subsequently subjected to a fatigue test in a pull-pull mode. This mode of stress was chosen so that the fracture would not be damaged during the fatigue process when the crack was opening and closing (because in this case the crack would always remain open) and especially during the final tensile failure. Thus the fracture surface could be examined in great detail, allowing the fatigue striations to be followed to their source with the use of the scanning electron microscope (SEM). In this way the crack initiator could be identified with certainty.

Date: 2015-12-11; view: 711